This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-295357, filed Oct. 18, 1999; No. 11-295358, filed Oct. 18, 1999; and No. 2000-130911, filed Apr. 28, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to an X-ray tube of a rotary anode type, particularly, to an X-ray tube of a rotary anode type equipped with a slide bearing of a dynamic pressure type lubricated by a liquid metal.
FIG. 1 is a cross sectional view showing a gist portion of a conventional X-ray tube of a rotary anode type equipped with a slide bearing of a dynamic pressure type and an X-ray tube apparatus having the X-ray tube housed in a housing. Reference numeral 141 shown in FIG. 1 represents a vacuum vessel of an X-ray tube of a rotary anode type. A cathode 140 for emitting an electron beam, a disc-like rotary anode target 142, etc. are arranged within the vacuum vessel 141. Also, an X-ray emissive layer 143 for emitting an X-ray is arranged in that region of the disc-like rotary anode target 142 which faces the cathode 140.
The disc-like rotary anode target 142 is fixed to a support shaft 145 by a fixing nut 144. The support shaft 145 is joined to a rotor 146 formed cylindrical as a whole. The rotor 146 is of a three-layer structure consisting of an outer cylinder 146a, an intermediate cylinder 146b and an inner cylinder 146c having a bottom. The support shaft 145 is joined to the intermediate cylinder 146b. 
A columnar stationary structure 147 is inserted into the inside of the inner cylinder 146c. A spiral groove 148 having a herringbone pattern is formed on the surface of the stationary structure 147, and a metal lubricant such as a Gaxe2x80x94Inxe2x80x94Sn alloy, which is in the form of a liquid at least during the operation of the X-ray tube, is supplied into a gap including the slide bearing section of a dynamic pressure type formed between the stationary structure 147 and the rotor 146 and into the spiral groove 148.
A lubricant storage chamber (not shown) for receiving the liquid metal lubricant is arranged in a central portion of the stationary structure 147. A plurality of lateral lubricant passageways or ducts are arranged in radial directions between the lubricant storage chamber and the slide bearing of the dynamic pressure type. The liquid metal lubricant housed in the lubricant storage chamber is supplied through the lubricant passageways into the slide bearing section of the dynamic pressure type.
The inner cylinder 146c of the rotor and the stationary structure 147, which collectively constitute the slide bearing of the dynamic pressure type, are arranged such that about 20 xcexcm of the bearing clearance can be maintained during the operation of the X-ray tube. Each of the inner cylinder 146c and the stationary structure 147, which collectively form the bearing surface, is made of a metal material such as an iron alloy tool steel, e.g., SKD-11 (JIS standards). The heat conductivity of SKD-11 is relatively small, i.e., 24 W/mxc2x7K at room temperature.
Two stepped portions 149, 150 are annularly arranged a certain distance apart from each other in the vertical direction in the outer circumferential portion of the stationary structure 147. The outer diameter of the stationary structure 147 is changed in each of the stepped portions 149 and 150 such that the diameter of the stationary structure 147 in the lower end portion positioned on the opposite side of the disc-like rotary anode target 142 is made smaller. A projecting portion 151 is annularly formed in the outer circumferential portion of the stepped portion 150 positioned in the lower portion. Also, a metal ring 152 is arranged on the outside of the projecting portion 151 in a manner to surround the stationary structure 147. Annular projecting portions 153 and 154 are arranged on the inner circumferential portion and the outer circumferential portion, respectively, of the metal ring 152. An outer edge portion 147a of the stationary structure 147 positioned in the lower portion in FIG. 1 extends to the outside of the vacuum vessel 141 so as to be utilized as a portion at which the X-ray tube of the rotary anode type is fixed to a housing 155.
The vacuum vessel 141 comprises a large diameter portion 141a made of a metal and surrounding the main portion of the disc-like rotary anode target 142 and a small diameter portion 141b surrounding the main portions of the rotor 146 and the stationary structure 147. The small diameter portion 141b is made of, for example, glass, and a seal ring 156 made of a thin metal body is bonded to the edge portion of the small diameter portion 141b. The tip portion of the seal ring 156 is hermetically welded to the tip portion of the projecting portion 154 on the outer circumferential portion of the sealing metal ring 152. Also, the tip portion of the projecting portion 153 in the inner circumferential portion of the sealing metal ring 152 is hermetically welded to the tip portion of the projecting portion 151 formed in the stepped portion 150 of the stationary structure 147. In this fashion, the stationary structure 147 is hermetically sealed to the vacuum vessel 141. A stator 157 serving to impart a rotating force to the rotary structure 146 is arranged on the outside of the small diameter portion 141b of the vacuum vessel 141. The stator 157 comprises an iron core and a coil wound about the iron core.
In the X-ray tube of the rotary anode type constructed as described above, the edge portion 147a of the stationary structure 147 is fixed to the bottom in the central portion of a pot-like holding member 158 made of an insulating material. In the holding member 158, the open edge portion of the cylindrical portion 158b is fixed to the housing 155 by a plurality of bolts 160. Also, a through-hole is formed in the central portion of the bottom of the holding member 158, and a top-shaped metal ring 158a having a central through-hole 159 is fixed to the bottom portion of the holding member 158 by a plurality of bolts 161. The outer edge portion 147a of the stationary structure 147 extends through the central through-hole 159 of the metal ring 158a. 
The outer diameter of the metal ring 158a is inwardly tapered toward the inside of the vacuum vessel 141 and an annular projecting portion 162 is formed in the inner circumferential portion in contact with the outer edge portion 147a of the stationary structure 147. Where the outer edge portion 147a of the stationary structure 147 is fixed to the metal ring 158a, the tip surface of the projecting portion 162 of the metal ring 158a is brought into contact with the stepped portion 150 of the stationary structure 147.
The outer edge portion 147a of the stationary structure 147 is fastened and fixed to the metal ring 158a by a nut 163 engaged with a male screw formed on the outer circumferential wall of the outer edge portion 147a of the stationary structure 147. In fastening the nut 163, the stationary structure outer edge portion 147a, which is to be fixed, is pulled downward in FIG. 1 so as to strengthen the contact between the tip surface of the projecting portion 162 and the stepped portion 160 of the stationary structure 147, with the result that the X-ray tube of the rotary anode type is fixed to the holding member 158.
A shielding member 164 shielding the X-ray and made of lead is arranged inside the housing 155 housing the X-ray tube of the rotary anode type. An insulating cooling oil is loaded in and circulated through the shielding member 164. Also, an X-ray radiation window 165 for taking out the X-ray to the outside is arranged in a region positioned sideward of the X-ray emissive layer 143. A circulating hole for circulating the insulating cooling oil is formed in the pot-like portion 158b and in the metal ring 158a of the holding member 158, and an inlet port 166 for introducing the insulating cooling oil is formed in the portion of the housing 155 positioned sideward of the holding member 159. The insulating cooling oil supplied through the inlet port 166 is allowed to flow through the clearance between the vacuum vessel 141 of the X-ray tube of the rotary anode type and the housing 155, as denoted by arrows Y.
In the conventional X-ray tube of the rotary anode type, the heat generated from the rotary anode target is transmitted from the anode to the vacuum vessel by radiation and, then, transmitted from the vacuum vessel into the insulating cooling oil so as to be dissipated. A part of the heat generated from the rotary anode target and the heat generated by the rotation of the slide bearing of the dynamic pressure type are conducted to, for example, the rotor constituting the anode rotary mechanism so as to be dissipated partly from the outer circumferential surface of the rotor. The remaining heat is further conducted via the bearing to the stationary structure and, then, to the stationary structure outer edge portion positioned outside the vacuum vessel so as to be dissipated to the outside of the tube.
It should be noted that the liquid metal lubricant consisting of, for example, a Ga alloy, which is supplied to the slide bearing section of the dynamic pressure type, is highly active. If the bearing section is heated to a high temperature, the liquid metal lubricant reacts with the metal material forming the stationary structure and the bearing surface of the rotor. As a result, a reacted metal layer is accumulated on the bearing surface so as to gradually decrease the depth of the spiral groove and the clearance or gap between the bearing surfaces, leading to deterioration of the rotary characteristics in some cases. It should also be noted that, if the bearing section is heated to a high temperature, a gas tends to be generated from various materials. What should be noted is that it is conceivable for the liquid metal lubricant to be pushed out of the bearing section by the gas bubbles thus generated so as to leak to the outside.
A measure for suppressing the temperature elevation in the rotor and the bearing section of the stationary structure is proposed in, for example, Japanese Patent Disclosure (Kokai) No. 7-130311. Specifically, it is proposed that the core portion of the stationary structure is made of a material having a high conductivity, and the heat transmitted to reach the stationary structure is further transmitted through the core portion of the stationary structure so as to be dissipated to the outside of the vacuum vessel. In this prior art, a molten metal, mainly a molten copper, is poured into the core portion of the stationary structure so as to form a body having a high heat conductivity. Naturally, it is difficult to manufacture the stationary structure. Also, the stationary structure is low in its mechanical strength.
It is also known to the art that heat dissipating fins are mounted to the outer edge portion of the stationary structure extending to the outside of the vacuum vessel of the X-ray tube of the rotary anode type, and an insulating oil is brought into direct contact with the heat dissipating fins for cooling the fins. Further, it is known to the art that a cooling medium is introduced into and circulated through a void formed inside the stationary structure so as to enhance the cooling efficiency.
However, in the cooling structure for cooling the outer edge portion of the stationary structure, it is difficult to obtain a sufficient heat dissipation effect because the outer edge portion of the stationary structure is considerably apart from the bearing section. On the other hand, in the structure in which a cooling medium is circulated through the inside of the stationary structure, the mechanical strength of the stationary structure is lowered because of formation of the hole reaching the inner region of the stationary structure.
Incidentally, the temperature of the slide bearing section of the dynamic pressure type is rendered nonuniform partly because a part of the heat generated from the rotary anode target is transmitted to the slide bearing section and partly because heat is generated from the bearing section itself. As a result, an undesired reaction tends to proceed between the liquid metal lubricant and the bearing surface in the high temperature portion.
An object of the present invention is to provide an X-ray tube of a rotary anode type, which permits preventing the temperature in the slide bearing section of the dynamic pressure type from becoming nonuniform and also permits suppressing the temperature elevation in the slide bearing section, which permits facilitating the manufacture and also permits increasing the mechanical strength of, particularly, the stationary structure, and which further permits maintaining stable rotary characteristics over a long period of time.
According to a first aspect of the present invention for achieving the object described above, there is provided an X-ray tube of a rotary anode type, in which at least one hole is so formed as to extend from an edge portion of a stationary structure along an axis of the stationary structure within a position avoiding a lubricant storage chamber and a lubricant passageway, and a heat transfer member having a heat conductivity higher than that of the stationary structure is inserted into the hole to form an integral structure.
According to a second aspect of the present invention for achieving the object described above, there is provided an X-ray tube of a rotary anode type, in which a rotor is formed of a plurality of cylindrical structures, and a heat transfer member having a heat conductivity higher than that of an inner cylindrical structure included in the plural cylindrical structures is bonded in a substantially cylindrical form to the outer circumferential wall of the inner cylindrical structure forming a slide bearing of the dynamic pressure type together with the stationary structure.
According to a third aspect of the present invention for achieving the object described above, there is provided an X-ray tube of a rotary anode type, in which at least one hole is formed to extend from an edge portion of a stationary structure within a position avoiding a lubricant storage chamber and a lubricant passageway, a heat transfer member having a heat conductivity higher than that of the stationary structure is inserted into the hole to form an integral structure, a rotor is formed of a plurality of cylindrical structures, and a heat transfer member having a heat conductivity higher than that of an inner cylindrical structure included in the plural cylindrical structures is bonded in a substantially cylindrical form to the outer circumferential wall of the inner cylindrical structure forming a slide bearing of the dynamic pressure type together with the stationary structure.
According to a fourth aspect of the present invention for achieving the object described above, there is provided an X-ray tube of a rotary anode type, in which a hole is formed to extend from an edge portion of a stationary structure within a position avoiding a lubricant storage chamber and a lubricant passageway, a heat transfer member having a heat conductivity higher than that of the stationary structure is inserted into the hole to form an integral structure, and a fluid passageway for circulating a cooling medium is formed in the heat transfer member.
Further, according to a fifth aspect of the present invention for achieving the object described above, there is provided an X-ray tube of a rotary anode type, in which a first portion of a stationary structure in which a slide bearing of a dynamic pressure type is arranged is formed of a predetermined first material, a second portion positioned farther from the rotary anode target than the first portion of the stationary structure is formed of a second material having a heat conductivity higher than that of the first material, and the first portion and the second potion are integrally joined to each other in a position avoiding the lubricant storage chamber and the lubricant passageway.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.